To form a film on a substrate (hereinafter, referred to as “wafer”) such as a semiconductor wafer, various methods such as a CVD (Chemical Vapor Deposition) method, an ALD (Atomic Layer Deposition) method are used. The CVD method includes supplying a raw material gas onto a surface of a wafer, and heating the wafer such that the raw material gas is subjected to a chemical reaction. The ALD method includes adsorbing an atomic layer or molecular layer formed of a raw material gas onto a surface of a wafer, supplying a reaction gas for oxidization and reduction of the raw material gas to produce reaction products, and repeatedly performing a sequence of these operations to deposit layers, which are formed of the reaction products, on the wafer. The above operations are performed by supplying the raw material gas into a reaction chamber which accommodates a plurality of wafers and is in a vacuum atmosphere.
Raw materials used in the CVD and ALD methods, when they are vaporized into raw material gas, often have a low vapor pressure. Such raw material gas is obtained by supplying a carrier gas into a raw material container that accommodates a liquid or solid raw material, followed by vaporizing the raw material into the carrier gas. However, in the raw material gas supplied by such a method, a vaporization amount (vaporization flow rate) of the raw material per unit time is varied over time due to a reduction of the raw material received in the raw material container, a change in residence time of the carrier gas inside the raw material container, or the like.
The constant vaporization flow rate of the raw material may be maintained by controlling a heating temperature of the raw material container or adjusting a flow rate of the carrier gas. However, when the heating temperature is controlled, it may result in poor responsiveness. In addition, when the flow rate of the carrier gas is adjusted, it may generate dusts as the flow rate of the carrier gas increases.
In the related art, there has been suggested a chemical vapor deposition apparatus which supplies a bubbling gas into a bubbler with a liquid raw material, bubbles the liquid raw material and supplies a raw material gas consisting of the bubbling gas and the bubbled raw material gas into a reaction chamber. In this apparatus, an internal pressure of the bubbler is controlled to be higher than an internal pressure of the reaction chamber, which prevents the raw material gas from flowing backward. However, this conventional apparatus is outdated in terms of a relationship between the internal pressure of the bubbler and the vaporization flow rate of the raw material.